MEMS Capacitive Bending and Axial Strain Sensor

ABSTRACT

A three-dimensional micro-electro-mechanical-systems (MEMS) capacitive bending and axial strain sensor capacitor is described. Two independent comb structures, incorporating suspended polysilicon interdigitated fingers, are fabricated simultaneously on a substrate that can displace independently of each other while attached to a substrate undergoing bending or axial deformation. A change in spacing between the interdigitated fingers will output a change in capacitance of the sensor and is the primary mode of operation of the device. On the bottom and to the end of each comb structure, a glass pad is attached to the comb structure to allow for ample surface area for affixing the sensor to a substrate. During fabrication, tethers are used to connect each comb structure to maintain equal spacing between the fingers before attachment to the substrate. After attachment, the tethers are broken to allow independent movement of each comb structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of non-provisional patent applicationSer. No. 11/552,547, of the same title filed 2006 Oct. 25 by the presentinventors, the disclosure of which is hereby incorporated by referenceherein in its entirety, and which claims the benefit of provisionalpatent Appl. No. 60/730,087, of the same title filed 2005 Oct. 26 by thepresent inventors, the disclosure of which is also hereby incorporatedby reference in its entirety.

FEDERALLY SPONSORED RESEARCH

This invention was made with government assistance provided by theNational Science Foundation under Contract No. BES-0097521. Thegovernment has certain rights to this invention.

BACKGROUND OF THE INVENTION Field of Invention

This invention relates to a strain sensor, specifically to a sensor thatcan measure axial and bending strain.

Background of the Invention

Variable capacitors play a fundamental role in high-frequency andradio-frequency (RF) circuits. In the last few years, MEMS variablecapacitors have drawn considerable interest due to their superiorelectrical characteristics, size and cost of manufacture.

While variable capacitors using MEMS technology can be readilyimplemented in standard semiconductor devices for applications inaerospace, consumer electronics and communications systems, researchershave attempted to provide application to medical systems or diagnostics.Modern medical science has emerged with a need to monitor physiologicalfunctions (i.e. intravascular pressure, intraocular pressure, etc.). Avariety of these monitoring devices require that their tasks beperformed wirelessly and implanted for indefinite terms to allow forpatient mobility, continuance of daily activities and avoidance ofcostly surgeries to remove the systems after utilization is complete.

An application of a MEMS bending and axial capacitive sensor is tomonitor strain changes of spinal instrumentation implanted during spinalfusion surgical procedures to assist orthopaedic surgeons withevaluation of fusion progression. The current method to assess fusion isthe evaluation of radiographic images. However, image obstructions oftenprevent a clear determination if fusion has occurred. A strain sensorcould be incorporated with a battery-less implantable telemetry systemand enclosed in a hermetically sealed package. After attachment to thespinal instrumentation, the sensor can vary its capacitance output dueto small changes in strain by the instrumentation as fusion occurs,thereby giving objective data to the orthopaedic surgeons of whetherfusion is occurring and potentially avoiding costly exploratory surgery.

Existing strain sensors are used to indicate the amount and the type ofdeformation (i.e. elongation or compression) of materials. These can beused to indicate the state of a material, predict material behavior orgather material properties. These types of sensors can gatherinformation in a variety of manners including changes in resistance andcapacitance. However, there has not been a sensor available that canmeasure bending and axial strain in a capacitive manner.

Inventors have developed sensors in attempts to measure bending strainin a capacitive manner. U.S. Pat. No. 5,827,980 to Doemens (1998) hasdeveloped a dual comb structure; however, the orientation of the deviceis situated at 45 degrees and is not meant to observe bending strain.Additionally by Doemens, U.S. Pat. No. 5,750,904 (1998) the inventor hasproposed a dual pair of comb structures that primarily measure axialforces or extension forces. However, bending is not the primary methodof actuation which would cause the comb structures to move verticallycausing a change in the overlapping surface area.

U.S. Pat. No. 6,606,913 to Gianchandani (2003) has disclosed a complexarray of elevated small comb structures or tines with verticalsidewalls. While undergoing axial strain, the tines will change theiroverlapping surface area, which can be correlated to change incapacitance and strain. However, due to the attachment method ofsecuring both ends of the comb structures to the substrate, the sensorwould not be able to actuate while undergoing bending strain. A similarcase is made with U.S. Pat. No. 7,035,083 to Lin (2006), where theattachment method of the tines will not allow vertical displacementallowing a change in the overlapping surface area of the tines.

Other U.S. Pat. Nos. 4,188,651 (1980), 4,941,363 (1990), 5,610,528(1997), 6,266,226 (2001) and 6,532,824 (2003) identify capacitive combstructures comprised on a thin film. However, the lack of verticaldimension or overlapping surface area can not be used to determine theamount of bending strain present in a deformed substrate.

Comb structures are prolific throughout the MEMS environment; however,very few are actuated by attachment via a substrate. Most examples areactuated by the electrostatic means of applying a voltage potential tothe independent comb structures as noted by Wu, U.S. Pat. No. 7,085,122(2006), Lin, U.S. Pat. No. 5,537,083 (1996), Gang, U.S. Pat. No.5,918,280 (1999), Muenzel, U.S. Pat. No. 5,723,353 (1998) and Nguyen,U.S. Pat. Nos. 6,236,281 (2001), 5,955,932 (1999), 5,839,062 (1998),5,491,604 (1996).

BRIEF SUMMARY OF THE INVENTION

A three-dimensional micro-electro-mechanical-systems (MEMS) bending andaxial capacitive sensor is described. Two independent comb structures,incorporating suspended crystalline interdigitated fingers, arefabricated simultaneously on a substrate that can displace independentlyof each other while attached to a substrate undergoing bending ordeformation. A change in spacing between the interdigitated fingersoutputs a change in capacitance of the sensor and is the primary mode ofoperation of the device. On the bottom and the end of each combstructure, a glass pad is attached to the comb structure to allow forample surface area to affix the sensor to the substrate using adhesives.During fabrication, tethers are used to connect each comb structure tomaintain equal spacing between the fingers before attachment to thesubstrate. After attachment, the tethers are broken to allow independentmovement of each comb structure.

Advantages of this system allow the sensor to use very low power sincethe method of actuation is mechanical. This allows the system to beincorporated in implantable and wireless medical device systems that donot require the use of a battery. Fabrication of the device does notrequire expensive silicon-on-insulator wafers and the sensor can easilybe incorporated into current semi-conductor fabrication processes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention,and, together with the general description of the invention given above,and the detailed description of the embodiments given below, serve toexplain the principles of the present invention.

FIG. 1 is a perspective view of the MEMS capacitive bending and axialstrain sensor before it is applied to a substrate and the tethers areunbroken.

FIG. 2 is a perspective view of the MEMS capacitive bending and axialstrain sensor after it is applied to a flat and non-bending substratewith the tethers broken.

FIG. 3A is a top view of the MEMS capacitive bending and axial strainsensor attached to a flat substrate not undergoing bending ordeformation with the tethers broken.

FIG. 3B is a top view of the MEMS capacitive bending and axial strainsensor attached to a bending substrate.

FIG. 3C is a side view of the MEMS capacitive bending and axial strainsensor attached to a bending substrate.

FIG. 4 is a perspective view of finite element modeling results of thedisplacement of the MEMS capacitive bending and axial strain sensor.

FIG. 5 is the front view of the testing fixture for the MEMS capacitivebending and axial strain sensor.

FIG. 6A is a top view of the interdigitated fingers of the fabricatedMEMS capacitive bending and axial strain sensor in the static positionwithout undergoing bending.

FIG. 6B is a top view of the displaced interdigitated fingers of thefabricated MEMS capacitive bending and axial strain sensor whileundergoing bending.

FIG. 6C is the capacitive output of the sensor while undergoing bendingin the testing fixture.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe drawings, in which illustrative embodiments are shown.

FIG. 1 is an perspective view of the three-dimensionalmicro-electromechanical-systems MEMS capacitive bending and axial strainsensor before it is affixed to a flat substrate in which the movablesilicon comb structures 1, 2 containing the suspended interdigitatedfingers are fabricated using conventional semiconductor fabricationtechniques. Each comb structure 1, 2 is elevated by an anchor 3, 4incorporated at the end the comb structure 1, 2. A glass pad 5, 6 isanodically bonded to the bottom of each anchor 3, 4 to provideelectrical isolation from the substrate and to increase the distancefrom the substrate to the plane of the interdigitated fingers to improvesensitivity. Electrically conductive epoxy 11, 2 is applied to the topof each anchor 3, 4. Electrical leads or wires 13, 14 are placed in theelectrically conductive epoxy 11, 12 and the assembly is cured in aheated oven at a specific temperature for an intended period of time.This enables the electrical leads or wires 13, 14 to maintain electricalcontinuity with the sensor via the electrically conductive epoxy andprovides a method to secure the electrical leads to the sensor. Tethers16, 17 are connected from the end of each comb structure to maintainequal spacing between the interdigitated fingers before application tothe substrate.

FIG. 2 is a perspective view of the three-dimensionalmicro-electromechanical-systems MEMS capacitive bending and axial strainsensor after it is affixed to a flat substrate 15 and the tethers 7, 8,9 and 10 have been broken. There is no bending or deformation of thesubstrate therefore the spacing between each of the fingers is equal onboth sides.

FIG. 3A is a top view of the three-dimensionalmicro-electromechanical-systems MEMS capacitive bending and axial strainsensor after it is affixed to a flat substrate 15 and the tethers 7, 8,9 and 10 have been broken.

FIG. 3B is a top view of the MEMS capacitive bending and axial strainsensor attached to a bending substrate 15. During bending, lateralmovement of each comb structure, as depicted by the arrows of movement,will cause the spacing between the interdigitated fingers to change andresult in an increase in capacitance. The capacitance relationship for aparallel plate system is given by

$\begin{matrix}{C = \frac{ɛ_{0}ɛ_{r\;}A}{d}} & (1)\end{matrix}$

where C is generated capacitance in farads (F) and ∈₀ is the dielectricof free space equal to 8.85×10⁻¹⁴ F/cm. The second dielectric constant,∈_(r), is the relative permittivity for the medium, which isdimensionless, between the two plates and is equal to 1 for air. Theoverlapping area between the two plates is A and d is the distancebetween the two plates. From this relationship, increasing or decreasingthe overlapping area of the plates will produce a linear difference incapacitance, whereas, adjusting the spacing between the two platesgenerates an inverse response.

FIG. 3C is a side view of the MEMS capacitive bending and axial strainsensor to a bending substrate. The shape of the bending substrateresults in a vertical displacement of the comb structures, which reducesthe amount of capacitance due to a reduction of the overlapping surfacearea between the interdigitated fingers, as supported by Equation (1).

FIG. 4 is a perspective view of the finite element modeling results ofone end of the MEMS capacitive bending and axial strain sensor. Themodel was simplified by showing only the vertical and axial displacementof one pair of interdigitated fingers in the comb structures 1, 2 toverify generated capacitance from calculations.

FIG. 5 is a front illustration of the testing fixture to verify thecapacitance output of the fabricated MEMS capacitive bending and axialstrain sensor. The testing fixture 16 applies loads to the substrate 15to induce bending on the substrate and to the attached MEMS capacitivebending and axial strain sensor.

FIG. 6A is a top view of the interdigitated fingers of the combstructures 1, 2 of the MEMS capacitive bending and axial strain sensorin the static conditions or not undergoing any axial or bending strain.

FIG. 6B is a top view of the interdigitated fingers 1, 2 of the MEMScapacitive bending and axial strain sensor undergoing bending in thetesting fixture from FIG. 5. A comparison of FIGS. 6A to 6B demonstratesmovement of the interdigitated fingers in the comb structures 1, 2,which result in a change in capacitance.

FIG. 6C is a graphical illustration of the capacitance output of theMEMS capacitive bending and axial strain sensor while undergoing bendingon the substrate 15 in the testing fixture 16.

By virtue of the present invention, a MEMS variable capacitor utilizesmulti-fingered interdigitated three dimensional comb structures to sensechanges in strain or deformation of the attached substrate. Themechanical method of actuation on a bending or axially deformingsubstrate provides certain advantages. A change in the capacitanceoutput of the device does not require a high voltage input to change thespacing and overlapping area of the interdigitated fingers, whichdelivers a change in capacitance.

As a result of the mechanical actuation of the device, relatively smallpower inputs, as compared to the prior art, are required to monitorcapacitive changes for an implantable medical device. This situationallows for a system that does not require a battery, which would have tobe recharged, monitored for leaks and eventually removed from the body.

A further aspect of the device allows the use of tethers to maintainalignment of the comb structures until application to a substrate. Thisalso allows for ease of shipping of the device to system manufacturersand placement onto a substrate when desired by the manufacturer.

An additional aspect of this device is fabrication of the device in away which does not require the use of expensive silicon-on-insulator(SOI) wafers to fabricate the comb structures with the inherentinterdigitated fingers. The device can be easily incorporated intocurrent semiconductor fabrication processes.

Another advantage of this device is that the number of interdigitatedfingers of the comb structures, their dimensions and the spacing betweenthem can be predetermined to define initial capacitance and sensitivityrequirements.

It should be appreciated that any patent, publication, or otherdisclosure material, in whole or in part, that is said to beincorporated by reference herein is incorporated herein only to theextent that the incorporated material does not conflict with existingdefinitions, statements, or other disclosure material set forth in thisdisclosure. As such, and to the extent necessary, the disclosure asexplicitly set forth herein supersedes any conflicting materialincorporated herein by reference. Any material, or portion thereof, thatis said to be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure material set forthherein will only be incorporated to the extent that no conflict arisesbetween that incorporated material and the existing disclosure material.

While the present invention has been illustrated by description ofseveral embodiments and while the illustrative embodiments have beendescribed in considerable detail, it is not the intention of theapplicant to restrict or in any way limit the scope of the appendedclaims to such detail. Additional advantages and modifications mayreadily appear to those skilled in the art.

1. A micro electromechanical system (MEMS) capacitive bending and axialstrain sensor, comprising: (a) a first comb structure, wherein the firstcomb structure comprises a first plurality of spaced apart members; (b)a second comb structure, wherein the second comb structure comprises asecond plurality of spaced apart members, wherein the first and secondpluralities of spaced apart members are arranged such that the first andsecond comb structures together form a capacitor to provide acapacitance value; and (c) a substrate, wherein the first comb structureand the second comb structure are independently mounted to the substratesuch that the first and second comb structures are movable independentlyrelative to each other; wherein the first and second comb structures areconfigured to provide a change in the capacitance value in response toaxial strain on the substrate; wherein the first and second combstructures are further configured to provide a change in the capacitancevalue in response to bending strain on the substrate.
 2. The MEMScapacitive bending and axial strain sensor as recited in claim 1,wherein the each of the first plurality of spaced apart members and thesecond plurality of spaced apart members comprise interdigitated fingerswith a vertical dimension that exceeds the width of the interdigitatedfingers.
 3. The MEMS capacitive bending and axial strain sensor asrecited in claim 2, wherein the interdigitated fingers each havesidewalls that form parallel plates of the capacitor.
 4. The MEMScapacitive bending and axial strain sensor as recited in claim 3,further comprising a dielectric medium disposed between each of theinterdigitated fingers.
 5. The MEMS capacitive bending and axial strainsensor as recited in claim 1, wherein the capacitance value is based atleast in part on the number of spaced apart members in the first combstructure and in the second comb structure.
 6. The MEMS capacitivebending and axial strain sensor as recited in claim 1, furthercomprising a first non-conductive structure interposed between the firstcomb structure and the substrate.
 7. The MEMS capacitive bending andaxial strain sensor as recited in claim 6, wherein the firstnon-conductive structure interposed between the first comb structure andthe substrate comprises glass.
 8. The MEMS capacitive bending and axialstrain sensor as recited in claim 6, further comprising a secondnon-conductive structure, wherein the second non-conductive structure isinterposed between the second comb structure and the substrate, whereinthe second non-conductive structure is discrete relative to the firstnon-conductive structure.
 9. The MEMS capacitive bending and axialstrain sensor as recited in claim 8, wherein the first comb structure iselevated above the substrate by the first non-conductive structure,wherein the second comb structure is elevated above the substrate by thesecond non-conductive structure.
 10. The MEMS capacitive bending andaxial strain sensor as recited in claim 1, wherein the first combstructure is electrically isolated from the substrate, wherein thesecond comb structure is electrically isolated from the substrate. 11.The MEMS capacitive bending and axial strain sensor as recited in claim1, wherein the sensor is formed of materials comprising silicon,silicon-dioxide, an electrically conductive epoxy, and glass.
 12. TheMEMS capacitive bending and axial strain sensor as recited in claim 1,wherein the substrate defines a first plane, wherein the first andsecond comb structures are arranged along a second plane, wherein thesecond plane is parallel to the first plane, wherein the first andsecond comb structures are configured to provide a change in thecapacitance value in response to axial strain on the substrate by movingrelative to each other in respective directions along the second plane.13. The MEMS capacitive bending and axial strain sensor as recited inclaim 1, wherein the substrate defines a plane when the substrate is ina non-bent configuration, wherein the first and second comb structuresare configured to provide a change in the capacitance value in responseto bending strain on the substrate by moving relative to each other inrespective directions that are not parallel to the plane.
 14. The MEMScapacitive bending and axial strain sensor as recited in claim 1,further comprising: (a) a first wire coupled with the first combstructure; and (b) a second wire coupled with the second comb structure.15. The MEMS capacitive bending and axial strain sensor as recited inclaim 14, wherein the first wire is coupled with the first combstructure by an electrically conductive epoxy; wherein the second wireis coupled with the second comb structure by an electrically conductiveepoxy.
 16. A micro electromechanical system (MEMS) capacitive strainsensor, comprising: (a) a first comb structure, wherein the first combstructure comprises: (i) an anchored end, (ii) a free end, and (iii) afirst plurality of spaced apart members located between the anchored endand the free end; (b) a second comb structure, wherein the second combstructure comprises: (i) an anchored end, (ii) a free end, and (iii) asecond plurality of spaced apart members located between the anchoredend and the free end, wherein the first and second pluralities of spacedapart members are arranged such that the first and second combstructures together form a capacitor to provide a capacitance value; and(c) a substrate, wherein the anchored end of the first comb structureand the anchored end of the second comb structure are mounted to thesubstrate such that the free ends of the first and second combstructures are movable independently relative to each other; wherein thefirst and second comb structures are configured to provide a change inthe capacitance value in response to one or both of axial strain orbending strain on the substrate.
 17. The MEMS capacitive strain sensoras recited in claim 16, wherein the free end of the first comb structureis positioned proximate to the anchored end of the second combstructure, wherein the free end of the second comb structure ispositioned proximate to the anchored end of the first comb structure.18. The MEMS capacitive strain sensor as recited in claim 16, furthercomprising: (a) a first pad positioned between the anchored end of thefirst comb structure and the substrate, wherein the first pad elevatesthe first comb structure over the substrate; and (b) a second padpositioned between the anchored end of the second comb structure and thesubstrate, wherein the second pad elevates the second comb structureover the substrate.
 19. A micro electromechanical system (MEMS)capacitive strain sensor, comprising: (a) a substrate; and (b) a dualcomb structure having a first end and a second end, wherein the secondend is opposite to the first end, wherein the dual comb structurecomprises: (i) a first comb structure, wherein the first comb structurecomprises: (A) a first set of fingers, and (B) a first anchor portionlocated at the first end of the dual comb structure, wherein the firstanchor portion secures the first comb structure to the substrate, and(ii) a second comb structure, wherein the second comb structurecomprises: (A) a second set of fingers, wherein the first and secondcomb structures are arranged such that the first set of fingers and thesecond set of fingers are interdigitated, wherein the interdigitatedfingers form a capacitor configured to provide a capacitance value, and(B) a second anchor portion located at the second end of the dual combstructure, wherein the second anchor portion secures the second combstructure to the substrate, wherein the first and second comb structuresare configured such that the spacing between the interdigitated fingerschanges in response to one or both of axial strain or bending strain inthe substrate to provide a change in the capacitance value in responseto one or both of axial strain or bending strain in the substrate. 20.The MEMS capacitive strain sensor as recited in claim 19, wherein thefirst comb structure further comprises a first free end opposite to thefirst anchor portion, wherein the first free end is positioned at thesecond end of the dual comb structure, wherein the second comb structurefurther comprises a second free end opposite to the second anchorportion, wherein the second free end is positioned at the first end ofthe dual comb structure, such that the first comb structure isconfigured to move relative to the substrate in a manner independentfrom movement of the second comb structure relative to the substrate.